a reference to a character variable containing one of the
above forms (see examples)

DEFAULT

If you specify the connection target literally (that is, not
through a variable reference) and you don't quote the value, then
the case-insensitivity rules of normal SQL are applied. In that
case you can also double-quote the individual parameters
separately as needed. In practice, it is probably less
error-prone to use a (single-quoted) string literal or a variable
reference. The connection target DEFAULT
initiates a connection to the default database under the default
user name. No separate user name or connection name may be
specified in that case.

There are also different ways to specify the user name:

username

username/password

username IDENTIFIED BY password

username USING password

As above, the parameters username and password may be an SQL identifier, an SQL
string literal, or a reference to a character variable.

The connection-name is used to
handle multiple connections in one program. It can be omitted if
a program uses only one connection. The most recently opened
connection becomes the current connection, which is used by
default when an SQL statement is to be executed (see later in
this chapter).

The last form makes use of the variant referred to above as
character variable reference. You will see in later sections how
C variables can be used in SQL statements when you prefix them
with a colon.

Be advised that the format of the connection target is not
specified in the SQL standard. So if you want to develop portable
applications, you might want to use something based on the last
example above to encapsulate the connection target string
somewhere.